1) Field of the Invention
The present invention relates to a technical field of an optical module such as a semiconductor laser device module that is used for an optical transmitter that transmits an optical signal. More particularly, this invention relates to an optical filter, a semiconductor laser device module, and a wavelength locker module that have a function of monitoring the wavelength of a laser beam that bears an optical signal transmitted in a wavelength division multiplexing (WDM) communication system.
2) Description of the Related Art
In recent years, the WDM has been focused as an element technique that is used to dramatically increase the quantity of transmission information, in the optical communications field. In this WDM communication system, wavelength intervals between multiplexed optical signals become extremely short. On the other hand, the wavelength of a semiconductor laser device that is used as a signal light source varies due to a temperature variation or the like. Therefore, there is a risk that the quality of signals is degraded due to the occurrence of crosstalk between adjacent optical signals. Consequently, because of the need for preventing the crosstalk between optical signals, the semiconductor laser device that is used in the WDM communication system has been required to have very high wavelength stability.
As a technique for realizing this wavelength stability, there is known a semiconductor laser device module that is structured to use a wavelength locker that detects a wavelength of an optical signal and feeds back the wavelength to a semiconductor laser device. According to this semiconductor laser device module, the oscillation wavelength of the semiconductor laser device is properly controlled based on a variation in the wavelength of the optical signal detected by the wavelength locker. Therefore, the semiconductor laser device module operates to keep the wavelength of an optical signal always stable.
FIG. 18 is a view that shows a structure of a semiconductor laser device module that includes a conventional wavelength locker. The semiconductor laser device module shown in FIG. 18 is constructed of a semiconductor laser device 101 that emits a laser beam, an optical fiber 102 that transmits a laser beam to the outside, parallel lenses 103 and 104 and a condenser lens 105 that are disposed as optical lenses to transmit a laser beam, a beam splitter 106 that branches a laser beam into two, a resonator 107 that has a function of selectively transmitting a laser beam having a predetermined wavelength band based on resonance, and a first photodetector 108 and a second photodetector 109 that receive laser beams and output optical detection currents. In the example shown in FIG. 18, constituent members are integrally fitted to a package of the semiconductor laser device module, and are held by a plurality of carriers (indicated as shaded portions).
In the above structure, a laser beam emitted from the front end (right side in the drawing) of the semiconductor laser device 101 is changed into a parallel beam by the parallel lens 103, and the parallel beam is condensed by the condenser lens 105. The condensed beam is incident to the optical fiber 102. On the other hand, a laser beam emitted from the rear end (the left side in the drawing) of the semiconductor laser device 101 is changed into a parallel beam by the parallel lens 104. The parallel beam is branched into two directions by the beam splitter 106 that is disposed in slanting relative to the light axis.
The laser beam branched to a direction in which the laser beam is transmitted through the beam splitter 106 passes through the resonator 107, and is received by the first photodetector 108. Further, the laser beam branched to a direction in which the laser beam is reflected by the beam splitter 106 is received by the second photodetector 109. The resonator 107 works to transmit a laser beam that has a specific wavelength component that is determined by a structure and characteristics of the laser beam.
A first optical detection signal output from the first photodetector 108 is compared with a second optical detection signal output from the second photodetector 109. Based on an output ratio of the two currents, a wavelength variation of the laser beam emitted from the semiconductor laser device 101 is detected. Based on a result of this detection, it is possible to control a laser beam to stabilize its wavelength, by giving a desired temperature change to the semiconductor laser device 101 with a Peltier device or by changing an injection current to the semiconductor laser device 101.
In the above conventional semiconductor laser device module, an optical filter that uses a Fabry-Perot etalon is known as a resonator of the wavelength locker. This optical filter is a device that forms a reflection surface on both sides of an optical transmission medium, and makes a transmitted laser beam generate a Fabry-Perot resonance. For example, an optical filter that uses quartz glass or crystal is used as an optical transmission medium.
However, according to the semiconductor laser device module that uses an optical filter, there is a problem of the occurrence of variation in the wavelength transmission characteristics of the optical filter due to changes in the environmental temperature. Specifically, a resonance wavelength varies depending on the change in the optical length of the optical transmission medium of the optical filter, and this generates a trouble in accurately controlling the wavelength with the wavelength locker. Particularly, when the optical filter using quartz glass is employed, as a variation in the optical length attributable to a temperature variation is large, the resonance wavelength also varies large due to the temperature. As a result, when the semiconductor laser device module is used in a situation in which the environmental temperature is changed, the precision of the wavelength locking is lowered. The reduction in the precision of the wavelength locking becomes serious particularly when the wavelength intervals between optical signals are short.